Indicator Test Function USDA Natural Resources Conservation Service P F W Soil Quality Indicators Soil Crusts Structural soil crusts are relatively thin, dense, somewhat continuous layers of non-aggregated soil particles on the surface of tilled and exposed soils. Structural crusts develop when a sealed-over soil surface dries out after rainfall or irrigation. Water droplets striking soil aggregates and water flowing across soil breaks aggregates into individual soil particles. Fine soil particles wash, settle into and block surface pores causing the soil surface to seal over and preventing water from soaking into the soil. As the muddy soil surface dries out, it crusts over. Left: Note the surface crust on this soil. The field was in tall fescue sod for 11 years. It was cleared and plowed using conventional Structural crusts range from a few tenths to as thick as two tillage methods. Photo courtesy Bobby Brock, USDA NRCS (retired). Right: Collected from a no-till field in Georgia’s Southern inches. A surface crust is much more compact, hard and Piedmont, good structure and aggregation are evident in the soil on brittle when dry than the soil immediately beneath it, the right. The same soil formed a structural crust under which may be loose and friable. Crusts can be described by conventional tillage. Note the sunlight reflectance of the crusted their strength, or air-dry rupture resistance. soil. Photo courtesy James E. Dean, USDA NRCS (retired). Soil crusting is also associated with biological and Dynamic - Management activities that deplete soil chemical factors. A biological crust is a living community organic matter and leave soil bare, smooth and exposed to of lichen, cyanobacteria, algae, and moss growing on the the direct impact of water droplets increase soil dispersion, soil surface that bind the soil together. A precipitated, surface sealing, runoff, erosion, and crusting. Excessive chemical crust can develop on soils with high salt content. tillage tends to break up soil clods into smaller sizes more susceptible to breakdown, bury most plant residue, and accelerate decomposition of organic matter. Harvest Factors Affecting methods that remove most or all of aboveground biomass Inherent - Soil crusting is related to soil texture, also prevent or reduce organic matter buildup and organic matter, and sodium content. Surface crusts are protection of the soil surface. Until the crop and its more common on fine-textured soils, such as silts, loams protective canopy is established, residue removal also and clays. In combination with the splashing effect of exposes soil to direct sunlight, which increases soil raindrops, increased runoff and erosion of fine-textured temperature and accelerates drying of the soil surface into soil increases the likelihood that a crust will develop. a hard crust. Crusts are usually thin and weak if present on coarse- textured, sandy soils. Relationship to Soil Function A surface crust indicates poor infiltration, a problematical Low organic matter results in poor soil structure, reduced seedbed, and reduced air exchange between the soil and pore space, and weak, unstable aggregates that fall apart atmosphere. It can also indicate that a soil has a high when raindrops hit them. Rainfall and runoff disperses the sodium content that increases soil dispersion when it is soil into individual particles that clog soil pores and seal its wetted by rainfall or irrigation. surface with a soil crust. Soils with high sodium content are more likely to develop Problems with Poor Function surface crusts since these soils are more readily dispersed Because they are hard and relatively difficult to break, with rainfall and irrigation. crusts restrict seedling emergence, especially in non-grass Helping People Help the Land... crops such as soybeans and alfalfa. Crusts can also reduce bulk density and increased pore space, and improves oxygen diffusion into the soil profile by as much as 50% if infiltration and water movement through soil. the soil crust is wet. Crust development soon after a crop is planted can result in such poor emergence that the crop Improved infiltration and water movement through soil might have to be replanted. decreases surface ponding and runoff, and helps protect soil from erosion. Good soil structure and aggregate Surface sealing and crusts greatly reduce infiltration, and stability are vital to supporting healthy, vigorous plants. increase runoff and erosion. Increased runoff results in less Healthy plants provide and conservation tillage methods water available in soil for plant growth. manage surface and subsurface plant residues needed to increase organic matter while maintaining and improving The sunlight (and energy) reflectance of a surface crust is higher than that of a non-crusted soil, so soil temperature aggregate stability and soil structure. may be lower and surface evaporation reduced where a crust exists (see photo on reverse). This could negatively To reduce the incidence of surface crusting of soils high in effect germination and development of healthy seedlings in sodium, irrigation water management prevents sodium cooler climates. accumulation at the surface, and gypsum (calcium sulfate) can be applied to promote flocculation and inhibit The relatively smooth surface of a crusted soil initially dispersion of soil particles. increases wind erosion of sandy soils. Loose sand particles blow across and abrade the smooth surface of the crust. It may be necessary to break a soil crust with a shallow, Roughening of the surface crust eventually reduces wind light tillage operation such as with a rotary hoe or row erosion. For soils with a small amount of sand, hard crusts cultivator, preferably when the soil is still moist. Light protect the soil surface from wind erosion. tillage can increase seedling emergence and help control weeds. Irrigation water can also be used to help with Surface crusts can have other limited benefits. Crusts seedling emergence. decrease water loss because less of their surface area is exposed to the air compared to a tilled, fluffy soil. In Conservation practices that minimize the development of a addition, a crust forms a barrier to evaporation of soil soil crust include: moisture. Reduced evaporation of soil moisture means more water remains in the soil for plant use. • Conservation Crop Rotation • Cover Crop Practices that lead to soil crusting include: • Residue and Tillage Management • Harvesting, burning, burying, or otherwise removing • Salinity and Sodic Soil Management plant residues and mulches so as to leave the soil surface bare for an extended period of time, and Measuring Soil Crusting • Soil disturbing activities that destroy organic matter, Crust air-dry rupture resistance can be measured by taking soil structure and aggregation, and result in very a dry piece of the crust about ½ inch on edge and applying smooth seedbeds. a force on the edge until the crust breaks. Generally, more force is required for crusts that are thick and have high Avoiding Soil Crusting clay content. A penetrometer to measure the penetration Practices reducing the development of soil crusts or resistance of the crust can be used. Crust thickness can also minimizing their negative impacts include those that be measured. protect or increase soil structure and organic matter and provide protective vegetative or residue cover on the soil Reference: Soil Survey Division Staff. 1993. Soil survey surface. No-till or reduced tillage of cropland is the best manual. Soil Conservation Service. U.S. Department of way to reduce or eliminate crust formation. If tillage is Agriculture Handbook 18. Chapter 3, Part 8. [Online] necessary, it should only be done to the minimum level http://soils.usda.gov/technical/manual/contents/chapter3d.html required for good seed germination and emergence. Large seeded crops do not require the same degree of clod size Specialized equipment, shortcuts, tips: reduction or as smooth of a seedbed as do small seeded crops. Residue intercepts the force of falling raindrops and A penetrometer may be needed to measure penetration is a source of organic matter. Organic matter stabilizes soil resistance of a soil crust. aggregates making them more resistant to the physical impact of raindrops. Improved aggregation results in lower Time needed: 30 minutes USDA is an equal opportunity provider and employer. June 2008.